Resisting culture is a lot easier than spreading it. Dropping a handful of capital ships can stop almost any hostile culture spread. If you're losing allegiance take into consideration that if there are multiple lanes connected to the planet, say 3 and 2 of them are dominated by hostile culture and are connected to your planet you will lose allegiance. However another alternative is you have met the advent superweapon which is a culture bomb. Also if it is advent their starbase may be adding addition spread / repel.